1. Field of the Invention
The present invention relates in general to optical fiber amplifier for amplifying optical signals in the field of optical communications, optical information processing and optical measurements and relates in particular to an apparatus for measuring a noise factor of an optical fiber amplifier.
2. Description of the Related Art
Presently, optical fiber amplifiers based on optical fibers doped with a rare-earth element, such as erbium (Er), are widely used, and are based on utilization of induced emission of inter-band energy release. Such optical amplifiers based on rare-earth element doped optical fiber (shortened to optical amplifier herein below) generate noise which is characterized by a noise factor. Methods for measuring the noise factor in an optical amplifier can be divided into two types: those based on electrical methods; and those based on optical methods. The electrical methods of determining the noise factor are based on injecting a high intensity modulated light into an optical amplifier whose output is monitored by a photodetector, and a ratio of noise to carrier signal strengths obtained by an electrical spectrum analyzer is used to calculate a noise factor. Optical methods of determining a noise factor are based on measuring the gain of an optical amplifier in relation to the intensity of spontaneous emission light generated within the optical amplifier.
The electrical methods of determining the noise factor rely, in general, on low noises from the associated measuring devices such as the photodetector, and furthermore, if there is some optical reflection in the measuring system, a problem called beat noise, which is dependent on the optical distance between the reflecting surfaces, makes a precise determination of the noise factor impossible. Optical methods in general, suffer from a problem that precision determination is not possible when high intensity light is injected into an optical amplifier, because in such a case, the optical spectrum analyzer measures the result of spontaneous emission light superimposed on stray light. There is a further problem common to both of these methods that all the methods require the use of expensive instruments such as electrical or optical spectrum analyzer.